This invention relates to halftone reproduction of a continuous tone original and, in particular, to a method and system for enhancing the number of gray levels of the original which are reproducible in the halftone reproduction.
Techniques for developing halftone reproductions of continuous tone originals are known in the art. In these techniques, the halftone original is formed from halftone dots of typically equal spacing whose size is varied to realize the various gray levels of the original.
In one technique for achieving a halftone reproduction, a screening process is used. In usual screening processes, gray level values of a periodic screen function (e.g., a repetitive triangle function) are added to the gray level values of the corresponding scanned elements (i.e. "pixels") of an original. Each sum value is then compared to a threshold value and a 0 (black) gray level value or a 255 (white) gray level value is generated depending upon the comparison. The resultant digital gray level values are then used to control a printer which darkens pixels in the reproduction corresponding to the O gray level values and leaves blank or white pixels corresponding to the 255 gray level values. The combined effect of these black and white reproduction pixels results in the halftone reproduction.
In present day practices, the screening process is often done electronically. U.S. Pat. No. 4,633,327 discloses one such electronic screening practice in which a sinusoidal screen function is employed. In this case, the threshold value used is adjusted so that over each period of the screen function the average value of the generated digital gray values equals the average value of the scanned pixel gray values. In this practice, the screen function is also adjusted in each screen period in dependence on the contrast of the pixel gray level values.
While the screening practice of the '327 patent and the screening practices of the other prior art systems provide adequate halftone reproductions, they are somewhat limited in the number of gray levels of the original which can be satisfactorily reproduced. As a result, there still remains a need for a halftone screening practice which can reproduce a larger number of gray levels of an original.
It is, therefore, a primary object of the present invention to provide a practice for developing a halftone reproduction which is capable of exhibiting a greater number of gray levels of the original.
It is a further object of the present invention to provide a halftone screning practice which enables the development of halftone reproductions capable of exhibiting gray levels over substantially the entire gray level range of 0 (black) to 255 (white).